1. Field of the Invention
The present invention relates to a servomotor control device, and specifically to a servomotor control device that enables a power cutoff circuit to be periodically tested during servomotor operation.
2. Description of the Related Art
For an emergency stop circuit included in a servomotor control device, a duplexed circuit is used to enhance safety, as disclosed in, for example, Japanese Laid-open Patent Publication No. 2004-237416. However, even in such a duplexed circuit, it is possible that a single failure may occur, which may lead to a multiple failure in a long-term continuous operation. Specifically, in an emergency stop circuit including a semiconductor element, it is highly possible that a short circuit failure in an output may occur due to an accident such as a temporary overvoltage and a short circuit or an accidental component fault, and therefore it is necessary to periodically test the emergency stop circuit in a short period of time.
Therefore, an emergency stop circuit including a semiconductor element is duplexed, and further a method for confirming periodically in a short period of time that the emergency stop circuit normally operates using a test pulse is used. In this case, it is possible to early detect a single failure of the emergency stop circuit and to safely stop the circuit using a circuit operating properly before a multiple failure occurs.
On the other hand, a method for safely performing a power cutoff of a servo amplifier using a semiconductor element used for motor power signals of the servo amplifier has been devised. FIG. 1 illustrates a conventional power cutoff circuit. The conventional power cutoff circuit includes a first photo-coupler pa that outputs a safety signal A, a second photo-coupler pb that outputs a safety signal B, a third photo-coupler pm that receives a monitor signal, a CPU 1001 that transmits the monitor signal to the third photo-coupler pm, an LSI 1002 that receives the safety signals A and B, a first switch SW1, a second switch SW2, an inverter circuit 1010 that drives a servomotor 200, a fourth photo-coupler p1 that controls an upper-stage semiconductor element 1011 of the inverter circuit 1010, and a fifth photo-coupler p2 that controls a lower-stage semiconductor element 1012 of the inverter circuit 1010. The reference signs 1003 to 1006 each represent a resistor. In the respective photo-couplers, the reference signs pa1, pm1, pb1, p11, and p21 each represent a light emitting element, and pa2, pm2, pb2, p13, and p23 each represent a light receiving element. The fourth photo-coupler p1 and the fifth photo-coupler p2 include diodes p12 and p22, respectively.
In this method, the safety signals A and B are input as an external signal that is not a pulse signal and is asynchronous with PWM (pulse width modulation) control. Therefore, it is necessary to perform a test of power cutoff circuit while PWM control is turned off, i.e., the servomotor 200 is stopped.
When it is possible to test a power cutoff circuit during servomotor operation, a failure of the power cutoff circuit can be early detected, and thereby a risk of a multiple failure is reduced and the reliability of a power cutoff circuit including a semiconductor element is enhanced.